This invention relates in general to electrical apparatus. More particularly, this invention relates to apparatus for shielding, an electrical circuit mounted on a circuit board, from electromagnetic interference.
Modern electrical equipment include many electrical circuits which are sensitive to external electromagnetic interference and radio frequency interference. This interference may originate from internal sources within the equipment or from external interference sources, such as power lines. Such interference can cause degradation or complete loss of important signals, rendering the electrical equipment inefficient or inoperative. Furthermore, governmental regulations (such as those of the Federal Communications Commission (FCC)), require that the levels of electromagnetic interference and radio frequency interference signals, produced by electrical circuits, be maintained below a predetermined level, so that such signals do not interfere with other electrical circuitry and equipment.
It is thus desirable to provide a shield (1) to protect electrical circuitry which is sensitive to electromagnetic intereference and (2) to contain EMI internally generated by electrical circuitry which produces electromagnetic interference. Where the electrical circuit to be shielded is provided on a circuit board, it has been proposed, in U.S. Pat. Nos. 4,386,388 and 4,661,888, to mount the circuit board in a metal enclosure. However, metallic enclosures are expensive, and are not suitable in applications where the external surface of the enclosure needs to be non-conductive. Thus, an additional non-conductive member surrounding the metallic enclosure is required when all-metal enclosures are used as a shield. It has also been proposed, in U.S. Pat. No. 4,567,317, to provide an enclosure of non-conductive material which has an internal surface of conductive material. As disclosed in the latter patent, the enclosure includes a base member and a complementary cover member, which are fitted together, to provide an enclosure for shielding a separate circuit board mounted within the enclosure. This technique is disadvantageous because of the expense and complexity of using two interfitting shield members, in addition to the circuit board to be shielded.
There is thus a need in the art, for apparatus for shielding, an electrical circuit mounted on a circuit board, from electromagnetic interference, which is simple, inexpensive and effective.